OCT Imaging
Optical Coherence Tomography (OCT) is a non-invasive, interferometric optical imaging technique that can generate micron resolution 2D and 3D images of tissue and other scattering or reflective materials. OCT is often used for biomedical imaging or materials inspection, and is part of a larger family of Laser Scanning Microscopy (LSM) techniques. First demonstrated for imaging the human eye imaging in 1991[Ref. 1], OCT has since been established as a clinical standard for diagnosing and monitoring treatment of eye disease. OCT has developed from operating in the time domain [Ref. 1], to now operating in the Fourier Domain [Refs. 2-6] with the depth information being frequency encoded in the captured optical interference fringe signals. Swept source OCT (SS-OCT) [Refs. 2-3] uses a wavelength scanning light source to illuminate an optical interferometer, and capture the time encoded interference fringe signals using a high speed detector. Spectral domain OCT (SD-OCT) [Refs. 4-6] uses a broadband source to illuminate an optical interferometer, and uses an optical spectrometer to record the spatially encoded interference fringe signals using a line-array detector (i.e. CCD or CMOS). SS-OCT and SD-OCT are two well documented Fourier domain OCT approaches. While this work will primarily describe systems using SS-OCT, it should be understood that other OCT techniques can be utilized with varying levels of success as each technique has its benefits and limitations. The Interferometer that is used to process the back reflected light and produce an interferogram is typical a Michelson design, but can be any of other design types also.
As an alternative technique to beam scanning OCT, Full-Field Optical Coherence Tomography (FF-OCT) is based on white-light interference microscopy [Ref. 7]. FF-OCM produces high resolution tomographic images in the en-face orientation by arithmetic combination of interferometric images acquired with an area camera and by illuminating the whole field-of-view using a low-coherence light source [Ref. 8].
OCT is capable of acquiring three-dimensional (3D) images of soft yet highly scattering biological objects typically with micron level resolution throughout the imaging volume. Typical data sets provide full volumetric details within an overall volume of approximately 10 mm (length)×10 mm (width)×3 mm (depth) when imaging translucent tissue objects like human skin. SS-OCT systems frequency encode the distance of an object using a reference delay located in the OCT instrument. The further an object feature is from the reference delay, the higher the frequency of the OCT interference fringe signals occur.
This work seeks to extend this relatively new technology to applications that are not limited to biological applications. A broad class of potential objects to be imaged is being defined as Engineering Objects, this term seeks to capture any object from natural or manmade, with an emphasis on manmade non-biological objects. While not directly excluding the biological objects this work presents an instrument and technology that is more appropriate for manmade Engineering Objects that range from being opaque to transparent. The term Engineering Objects will be used to refer to this general class of objects.
When the objects are made from materials that don't allow light to penetrate the surface, and at the same time reflect a measurable fraction of a light, then these objects will be called Reflective Objects. Here a measurable fraction is quite loose, but assumed to be in the range of 0.1% to 100%. An example of a Reflective Object could be a freshly machined aluminum precision mechanical part with a surface finish that is of sufficiently high quality so as to reflect a portion of an incoming laser beam. With the remainder of the light being scattered.
There are a number of technical challenges that stand in the way of applying what's been learned from the application of OCT to the biological sciences to the broader class of Engineering Objects. The foremost challenge has been the limited depth range over which most OCT systems operate, typically in the range of 5 to 20 mm range. This limitation comes from the coherence length limitation of the swept source or the spectral resolution limitation in an optical spectrometer. This limited measurement range meant that the reference surface in an OCT system would need to be positioned with optical delay difference in a few tens of millimeters from the object surface, in order to capture a 3D digital image of an object.
Another limitation is the previously mentioned frequency encoded signaling scheme inherent in SS-OCT, wherein the longer the distant of an object feature the higher the frequency of OCT signals. Currently swept source lasers are capable of operating at well over 100 kHz [Ref 9], and an analog to digital (A/D) converter needs to operate in the range of 500 MS/s (Mega Samples per second) when a ranging depth of 10 mm is acceptable. It will be shown that scaling up the ranging depth while also going to higher laser scan rate requires extremely high A/D converter speeds which significantly increases system cost.
This work will present a series of adaptations to the OCT based instruments that are designed for the life science applications. These new instruments will be shown to operate in a diverse range of industrial applications. It is also understood that bringing some of these new techniques back to the life sciences will offer improvements within that market, ranging from enhanced robotic surgery, to whole body scanning of living organisms.
The first illustration of an industrial application of this new technology will make efficient use of very high speed swept lasers in metrology applications. An OCT enhanced coordinate measuring machine can be used as a metrology tool in machine shops that produce precision metal parts using machine tools.
A coordinate measuring machine (CMM) is a well-known device that is well suited to measuring the dimensions of objects that aren't too complex and can fit within the object volume of the CMM. More specifically, the large family of parts encompassed in the subset of Reflective Objects made from aluminum and stainless steel is considered. A conventional CMM uses a mechanical probe with electronic touch trigger to send the X, Y, Z coordinate information about the Engineering Object to the user and/or a computer. The older touch probe CMM machines would be used to verify a number of critical dimensions on a machined part to validate a prototype or a production setup. Much as a machinist would spot check a relatively small numbers of points on a machined part to ensure it is made to the tolerances specified in the corresponding mechanical drawing or solid model. The most common CMMs use a gantry type structure with electronic motion control that include both a capability to power a touch probe as well as a capability to precisely locate the probe in space.
Once an object is in production the CMM could be used to periodically test this relatively small number of data points as compared to the typically millions or more data points required to digitally render a part with high fidelity. Newer CMMs utilize drag probes that take data points at regular intervals providing for a higher density of points, needed for the more complex shapes possible with CNC machine tools.
A CMM can be thought of a class of robots within a broader range of industrial robots that perform geometrical measurement of the Engineering Objects. When a CMM is referenced its meaning typically includes a touch probe with a Stylus that contacts the surface via a hardened ball, ruby or tungsten carbide. The touch probe is capable of making measurements relative to a laboratory coordinate system. The CMM also offers a teach mode where the user takes a set of measurements of a “golden object”, the system then repeats this set of measurements on subsequent objects.
The CMM has historically been an instrument that was used to verify critical dimensions, much as a machinist might use a “machinist micrometer” to spot check a feature on a part during or after machining. As non-contact CMM probes have become available, dense whole part scanning has become possible, but not typically required on the shop floor. If an Engineering Object has free-form geometry then dense sampling is more likely to be beneficial. The CMM approach is often adequate if there is sufficient prior knowledge about the Engineering Object and its features to be inspected, allowing system to plan an optimized measurement path, especially if the actual features do not deviate too much from what's expected.
Many non-contacting methods are recently available that improve the speed while maintaining the resolution and accuracy expected for a CMM. These methods include single point laser triangulation [Ref. 10], laser line scanning [Ref. 11], structured-light illumination [Ref. 12], chromatic white light scanning [Ref. 13], and single wavelength confocal imaging [Ref. 14]. Some non-contact measurements are typically inferior to contact measurements in resolution and accuracy but offer higher measurement speed. Recently the non-contact Confocal microscope systems have shown both improved speed and resolution, however because a high numerical aperture objective is required to ensure high spatial resolution, the working distance of a confocal microscope is often limited to a few millimeters. More recently, non-contact CMM probes based on optical interference techniques including optical coherence tomography method [Refs. 15, 16] have been proposed.
Many CMMs utilize a mechanical probe that makes physical contact with the Engineering Object. In order to measure the Engineering Object, the probe needs to be positioned in X, Y, and Z directions relative to the Engineering Object, and then it needs to contact the Engineering Object. This movement of mechanical elements, followed by probe contact is a time consuming process. A SS-OCT based CMM (SS-CMM) would be capable of making over 1,000,000 points per second, a speed not possible with contact or current non-contact CMMs.
The unique frequency encoding of distance allows the SS-CMM to take data with a large degree of flexibility in the actual distance between the Engineering Object and the position of the CMM probe. This freedom allows the SS-CMM to take data points even at irregular separations between the Engineering Object and the location of the probe. The limit to this feature of the SS-CMM is the laser beam divergence that increases as the beam waist is made smaller, a useful concept that captures this feature is the Rayleigh Range of a focused beam. The Rayleigh length is the distance over which the beam waist varies by the square root of 2 or approximately 1.414 times. This Rayleigh length is a useful parameter as it can be used to quantify the working distance of a SS-CMM. The SS-CMM can be outfitted with a dynamic focusing control mechanism that can rapidly shift the location of the beam waist.
With the common gantry style CMMs the measurement volume is defined by the limits of the movement in the X, Y, and Z directions. This is a limitation that would be greatly reduced by use of a SS-OCT based optical probe as it can scan areas outside the confines of a fixed measuring envelope using scanning laser beams.
The mechanical probe of a CMM is a delicate accessory that is prone to wear. The non-contact SS-OCT probe would eliminate wear. Many mechanical probes are delicate accessory that needs to approach the Engineering Object being mapped nearly perpendicular to the point that is being objected. The non-contact SS-OCT probe can be configured to overcome this restriction.
The object surfaces need to be rigid and have low contact force with the probe, this sets an upper limit on the speed with which the trigger point in a touch probe can be achieved. This same issue does not apply to a SS-CMM.
Delicate parts that do not permit single point contacting cannot be inspected with a mechanical probe. Additionally scanning mechanical probes pose an even greater risk of damaging the Engineering Object being measured. Hence the need for non-contacting solutions that have more recently been developed. These newer methods include: single point laser triangulation, laser line scanning, confocal imaging, chromatic white line scanning (a form of confocal imaging), digital camera based structured illumination, and optical interferometry based non-contacting probes.
The single point laser triangulation method requires a laser source and a position sensitive detector. The limitations are measurement accuracy that depends on the size of the instrument: longer baseline length between the source and the detector gives higher accuracy. Installing this type of instrument in commercial systems often requires a space larger than ideal. Also the relationship between detector output signal and object distance is not linear.
The laser line scanning, or more generally the structured illumination method, projects lines or 2D patterns on the object surface, and captures the line shape introduced by the object surface using a 2D digital camera. By scanning the laser line across object surface, or illuminating the surface with a grid pattern, the 3D surface profile of the object can be measured. This method is relatively fast however the measurement results largely depend on the illumination technique: both the transverse and depth resolution of the measurement depend on the width of the laser illumination line, and the size of the object that can be scanned is also limited by the illumination scheme used. Another drawback of this technique, is the limited dynamic range of the measurement, the Engineering Objects that can be 3D Digitized using this technique are limited in twits of their optical and physical properties, for example if the Engineering Object has large variations in reflectivity or largely scatters the illumination beams.
The chromatic white line scanning method leverages the principle introduced by Marvin Minsky in 1957 known as the confocal imaging. In chromatic white line scanning the instrument focuses different wavelengths from a white light source, at different focal depths. This technique uses a wavelength dependent focal length optical system. The object's surface location can be calculated by measuring the wavelength of reflected light passed through a confocal aperture. This method can measured very high transverse (<1 μm) and depth (a few nm) resolution about object positions, due to the confocal principle used. However, in this measurement approach the object's surface being imaged needs to be located very near the focal distance of the confocal objective, and the maximum object distance that can be measured is limited by the instruments ability to position the confocal measurement head at a precisely fixed distance above the object surface.
The OCT based 3D surface measurement methods have been developed in recent years. In EP 1 744 119, an apparatus for surface measurement using optical interference signals generated by a frequency swept source is disclosed. In U.S. Pat. No. 9,127,929, a frequency swept source based CMM is equipped with a probe capable of beam rotational control, allowing perpendicular beam impingement on object surface to improve the measurement accuracy. The swept sources disclosed in above examples are of external cavity type or fiber ring type tunable lasers, with wavelength selective element external to the gain media of the laser. These types of swept sources have proven to support maximum coherence length of a few tens of mm. When measuring industrial work pieces with height variation orders of magnitude larger than coherence length of the swept source, the probe needs to move to close to the surface, or to follow the height variations of the object, in order to generate OCT interference fringe signals with sufficient fringe contrast and signal-to-noise ratio for measuring surface height information. This means frequent probe position adjustment is needed which can significantly slow down the speed of the system measuring a large object.
Since the first demonstration of a fast, broadband MEMS tunable VCSEL for OCT imaging in 2011 [Ref. 9], swept source OCT powered by MEMS tunable VCSEL have achieved a combination of ultrahigh sweep speeds, wide spectral tuning range, flexibility in sweep trajectory, and extremely long coherence length [Refs. 17-18], which cannot be simultaneously achieved with other swept source technologies to date. The unique laser cavity design of MEMS tunable VCSEL of micron-scale cavity length enables single mode operation without mode hopping. Consequently, the coherence length of the laser can be extremely long. The sweeping of output wavelength of the VCSEL is the inherent result from the laser cavity change by moving the MEMS mirror that defines the cavity length together with another fixed mirror. This wavelength selection mechanism is fundamentally different from all other types of swept sources including external cavity tunable lasers and fiber ring tunable lasers. The long coherence length and ultra-high speed of MEMS tunable VCSEL brought exciting new applications as well as new challenges to the OCT system design, because all conventional SSOCT systems have a finite maximum electronic detection bandwidth for their detector systems, trade-off must be made when optimize the system for either high depth resolution, long depth measurement range, or high measurement speed. Another challenge is related the optimal transverse resolution of the system, to maintain minimum optical beam spot size over the very long depth measurement range, since the object surface can be at arbitrary height. Techniques are disclosed to overcome above limitation and challenges, as detailed in the following part of this application. The adaptation of MEMS tunable VCSEL powered swept source OCT system for a coordinate measurement machine and its application in measuring industrial objects are also disclosed.
Understand the Measurement Optical Resolution
In an OCT system, unlike other laser scanning imaging system, the optical resolution in the X and Y dimensions (see FIG. 1) are decoupled from the optical resolution in Z (the depth direction), as they are determined by different physical properties of the optical system, hence transverse resolution and depth resolution are separately discussed.
Recalling that the basic OCT measurement consists of an A-Scan which can locate surfaces over a range of depths simultaneously, not just at the focal point of the objective. The transverse resolution however is smallest at the waist of the focused OCT laser beam, and is determined by the wavelength and focusing condition of the objective lens (see Eq. 1). Typically in an OCT system the transverse resolution varies slowly with depth, and is a minimum at the beam waist (focus), becoming coarser as the depth increases or decreases about this waist depth. Gaussian Beam Optics is applicable here, and shows how a system can trade off resolution at the beam waist, for a smaller variation across the entire depth range of interest. The details are explored below, starting with Equation 1 and FIG. 1, combined they show the relationship between the objective lens used and the resulting shape of the probe beam.
                              Δ          ⁢                                          ⁢          x                =                              Δ            ⁢                                                  ⁢            y                    =                      1.22            ⁢                                          f                ⁢                                                                  ⁢                                  λ                  0                                            D                                                          (                  Eq          .                                          ⁢          1                )            
In Eq. 1, λ0 is the center wavelength, D is the diameter of the laser beam as it enters the objective lens, and f is the focal length of the objective lens, this formula assumes that the laser beam quality is close to ideal as typically is the case for Swept Source Lasers that are coupled into single mode optical fiber (as is standard practice in the biomedical applications of SS-OCT).
The depth range is defined as the 2ZR centered at the waist of the beam. For example a 10 mm focal length lens and a 1 mm diameter 632 nm laser beam the waist diameter will be about 8 um, and 2ZR will be equal to about 160 um. Using the same 1 mm diameter laser, but changing the focal to 100 mm then the waist diameter will be approximately 80 um and 2ZR will equal 16 mm.
In an OCT System the depth resolution is independent of the transverse resolution, with the depth resolution remaining constant along the full depth range of the instrument. Assuming a Reflective Object, the depth range is the distance over which the light from the object and the reference light can constructively interfere with sufficient contrast to produce a signal with a SNR sufficient to ensure detection.
The calculated depth resolution is inversely proportional to the bandwidth of the light source. Assuming a Gaussian spectral shape of the light, the theoretical depth resolution Δz, is given by:
                              Δ          ⁢                                          ⁢          z                =                  0.44          ⁢                                    λ              0              2                                      Δ              ⁢                                                          ⁢              λ                                                          (                  Eq          .                                          ⁢          2                )            
where λ0 is the center wavelength and Δλ is the full width half max (FWHM) of the light source spectrum. Table 1 shows the theoretical transverse and depth resolution values for some common OCT systems, where the assumed Δλ is typically about 5% of the center wavelength given as λ0, with a nearly Gaussian shape spectral profile:
TABLE 1Calculated OCT system resolution valuesOCT source center wavelengthnm85010501300OCT source spectral bandwidthnm506060(−3 dB)Focal length of the objective lensmm101010beam diametermm333Transverse resolutionmicron3.464.275.29Depth resolutionmicron6.48.112.4
Biomedical OCT images are constructed using the intensity information of the interference fringe signals, this provides an imaging capability that includes the interior as well as exterior surfaces of the entire 3D volume. In this context, the depth resolution can be understood as the minimum distance between two reflection surfaces in the depth direction that can be distinguished in an OCT image. As seen from the calculation in Table 1, the depth resolution is limited to a few microns for common OCT systems. Higher resolution is often required in material surfaces analysis, such as measuring the surfaces of machined metal parts or surfaces of optical mirrors. While the depth resolution can be enhanced by combining multiple swept source lasers, it will be apparent that for the purposes of examining Reflective Objects the resolution can be greatly enhanced by calculating the centroid of the signal that results from an isolated surface.
Understand the Relationship Between Speed, Resolution, and Depth
MEMS tunable Vertical-Cavity Surface-Emitting Lasers (VCSELs) are an ideal source for SS-OCT imaging, optical metrology, and spectroscopy applications because of their micron scale laser cavity length. Low mirror mass enables high sweep speeds greater than one million scans per second. The VCSEL design supports single mode operation without mode hops while scanning at high repetition rates. The single mode operation of the laser permits coherence length longer than meters during the fast wavelength sweep, and high quality interference fringe signals generated from optical interferometers of long delays.
In swept source OCT, the maximum depth of reflection that can be measured by the system is determined by two factors: the coherence length of the source and the electronic frequency bandwidth of the detection system. The coherence length corresponds to the optical delay in the interferometer at which the interference fringe contrast drops to 50%. Beyond the coherence length of the source the measurement sensitivity of a swept source OCT system drops significantly. As for the detection system limitation, SS-OCT frequency encodes the depth information, hence it is important that the detection system must have sufficient bandwidth to capture the high frequency signals that result from long standoff distances combined with high speed wavelength scanning of the VCSEL.
More formally, the coherence length of a swept source is defined as the averaged instantaneous coherence length over the entire wavelength range of the source. Because swept source does not emit light at a constant wavelength, instead the output wavelength changes as a periodic function of time. The instantaneous coherence length is given by:
                              L          c                =                                            2              ⁢                                                          ⁢              ln              ⁢                                                          ⁢              2                        π                    ⁢                                    λ              2                                      Δ              ⁢                                                          ⁢              λ                                                          (                  Eq          .                                          ⁢          3                )            
where λ is the instantaneous output wavelength and Δλ is the instantaneous line width of the source respectively. In experiment, the coherence of the swept source is measured as the path length difference in the two interferometer aims where the interference fringe contrast drops to 50% of the fringe contrast at zero path length difference. A long coherence swept source like the VCSEL can generate high contrast interference fringe signals from long interferometer delays. The coherence length is an important specification for a swept source laser because it imposes a limitation on depth measurement range from the source side. It should be noted that while the 50% fringe contrast test provides practical information as to the finite range over which a SS-OCT will work, it is important to remember that various applications will in some cases be less sensitive to this 50% point, such as in the case with a reflective metal object machined to generally accepted surface finishes.
The bandwidth of the complete detection system is an important element in a SS-CMM, the system consisting of the photo detectors, amplifiers, and data acquisition devices. The response time of the photo detectors, frequency bandwidth and slew rate of the amplifiers, and the object rate of the data acquisition device all affect the detection system's ability to measure the high frequency interference fringe signals. Typically the frequency response of a detection system degrades at higher frequencies. Therefore, it is important to control the frequency bandwidth of the interference signals to match the optimal frequency response range of the detection system.
The electronic frequency of a signal coming from a depth D is given by:
                              f          ⁡                      (            D            )                          =                                            2              ·                              R                Ascan                            ·              Δ                        ⁢                                                  ⁢                          λ              ·              D                                            λ            0            2                                              (                  Eq          .                                          ⁢          4                )            where λ0 is the center wavelength, Δλ is the FWHM of the wavelength sweep range of the swept source, and Rscan is the repetition rate of the laser.
For a system operating with a maximum depth Dmax of 10 mm and a laser repetition rate of 100 kHz, a center wavelength of 1 μm and a FWHM tuning range of 50 nm yields a signal bandwidth f(10 mm)=108 Hz=100 MHz.
Because the interference fringe signals coming from zero delay is a DC signals, the required detection bandwidth is from DC to f(Dmax). Required by the sampling theorem, the sampling rate (Sa) of the data acquisition device is:Sa≥f(Dmax)  (Eq. 5)
Using Eq. 5 for this example system the minimum sampling rate would need to be 200 MHz. While commercial DAQ cards (ATS9360 by Alazar Technologies Inc., Canada) are available with GHz sampling rate it would be advantageous to manage the system such that the maximum sampling rate is controlled without sacrificing the overall performance of the system. Reducing the bandwidth of the detection system not only reduces the total cost of the system, but also limits the total amount of broadband noise that is seen by the detectors.
For a particular detection system where the maximum detection bandwidth f(Dmax) is a constant, the relationship between the maximum measurement depth, depth resolution and the swept rate of the source become:
                                                        R              Ascan                        ·            D                                Δ            ⁢                                                  ⁢            z                          =        const                            (                  Eq          .                                          ⁢          6                )            
Therefore, trade-off must be made if a swept source OCT system is applied for measurement requiring different speed, spatial resolution and depth range.